Transmit circuit, method for adjusting a bias of a power amplifier and method for adapting the provision of a bias information

ABSTRACT

A transmit circuit includes an envelope tracker configured to determine an envelope of a transmit signal and provide bias information based on the determined envelope of the transmit signal. The transmit circuit further includes a power amplifier configured to generate an RF output signal based on the transmit signal, a bias provider configured to provide a bias for the power amplifier based on the bias information, and an impedance determinator configured to determine a measure of a load impedance of a load coupled to an output of the power amplifier. The envelope tracker is configured to adapt the bias information based on the measure of the load impedance.

REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 13/191,685filed on Jul. 27, 2011.

FIELD

Embodiments of the invention relate to a transmit circuit, a method foradjusting a bias of a power amplifier and a method for adapting theprovision of bias information. Further embodiments of the inventionrelate to a control of power amplifier output power headroom in anantenna tuner based system.

BACKGROUND

Conventional methods for adjusting a bias of a power amplifier comprise,for example, methods for reducing a bias current of the power amplifierbased on a detected output power level. Such a reduction of the biascurrent is performed by using a conventional bias control approach. Thebias control comprises, for example, a control of a driver for changingthe supply voltage of the power amplifier based on certain inputparameters. Conventional mobile communication devices including such abias control are based, for example, on Wideband Code Division MultipleAccess (WCDMA). WCDMA describes a multiple access method, whileUniversal Mobile Telecommunications System (UMTS) is the standard basedthereon.

SUMMARY

Embodiments of the invention provide a transmit circuit, wherein thetransmit circuit comprises a power amplifier for amplifying an RF inputsignal to obtain an RF output signal, and an antenna tuner fortransforming an antenna impedance to an impedance at an input of theantenna tuner, wherein the input of the antenna tuner is coupled to anoutput of the power amplifier. The transmit circuit further comprises abias controller for controlling a bias of the power amplifier. The biascontroller is configured to provide a bias control signal to adjust thebias of the power amplifier based on a determination of a measure of aload impedance provided to the power amplifier by the antenna tuner.

Embodiments of the invention provide a transmit circuit, wherein thetransmit circuit comprises a power amplifier for amplifying an RF inputsignal based on a supply voltage to obtain an RF output signal, and anantenna tuner for transforming an antenna impedance to an impedance atan input of the antenna tuner, wherein the input of the antenna tuner iscoupled to an output of the power amplifier. The transmit circuit alsocomprises a bias controller for controlling a bias of the poweramplifier. The bias controller comprises an impedance determinator fordetermining a measure of a load impedance provided to the poweramplifier by the antenna, a bias information provider for providing abias information in dependence on the measure of the load impedance, anda DCDC converter for adjusting the supply voltage of the power amplifierbased on the bias information. The bias information provider isconfigured to provide the bias information such that a parameter of theRF output signal lies within a predefined range for a plurality of loadimpedances determined by the impedance determinator. The bias controlleris also configured to: provide a first bias control signal to set thebias of the power amplifier to a comparatively high level during aninitial transmission time interval or after an occurrence of a frequencychange in a hopping sequence, determine the measure of the loadimpedance provided to the power amplifier by the antenna tuner, providea second bias control signal that is different from the first biascontrol signal to adjust the bias of the power amplifier to acomparatively lower level based on the determination of the measure ofthe load impedance for a consecutive time interval, and increase thebias level of the power amplifier by providing an increased bias controlsignal in response to a detection of a change of the measure of the loadimpedance which exceeds a predefined threshold. Additionally, the biascontroller is configured to set the bias of the power amplifier to thecomparatively high level as long as a current measure of the loadimpedance is unavailable or as long as the load impedance presented tothe power amplifier by the antenna tuner has not reached a predeterminedimpedance region. Furthermore, the bias controller is configured toreduce the bias of the power amplifier as soon as the load impedancepresented to the power amplifier by the antenna tuner is brought intothe predetermined impedance region by the antenna tuner.

Embodiments of the invention provide a transmit circuit, wherein thetransmit circuit comprises an envelope tracker for determining anenvelope of a transmit signal and for providing bias information basedon the envelope of the transmit signal. The transmit circuit alsocomprises a power amplifier for obtaining an RF output signal based onthe transmit signal, a bias provider for providing a bias for the poweramplifier based on the bias information, and an impedance determinatorfor determining a measure of a load impedance of a load coupled to anoutput of the power amplifier. The envelope tracker is configured toadapt the provision of the bias information based on the measure of theload impedance.

Embodiments of the invention provide a method for adjusting a bias of apower amplifier. The method comprises amplifying an RF input signalusing a power amplifier to obtain an RF output signal, and transformingan antenna impedance to an impedance at an input of an antenna tuner,wherein the input of the antenna tuner is coupled to an output of thepower amplifier. The method also comprises controlling the bias of thepower amplifier. Controlling the bias comprises providing a bias controlsignal to adjust the bias of the power amplifier based on adetermination of a measure of a load impedance provided to the poweramplifier by the antenna tuner.

Embodiments of the invention provide a method for adapting the provisionof bias information. The method comprises determining an envelope of atransmit signal, providing a bias information based on the envelope ofthe transmit signal, and obtaining an RF output signal based on thetransmit signal using a power amplifier. The method also comprisesproviding a bias for the power amplifier based on the bias information,determining a measure of a load impedance of a load coupled to an outputof the power amplifier, and adapting the provision of the biasinformation based on the measure of the load impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments according to the present invention will subsequently bedescribed in reference to the enclosed figures in which:

FIG. 1 shows a block diagram of an embodiment of a transmit circuitincluding an antenna tuner and a bias controller;

FIG. 2 shows a block diagram of a further embodiment of a transmitcircuit including an antenna tuner and a bias controller with animpedance determinator, a bias information provider and a DCDCconverter;

FIG. 3 shows a block diagram of a further embodiment of a transmitcircuit including an antenna tuner and a bias controller with animpedance information look-up table;

FIG. 4 shows a block diagram of a further embodiment of a transmitcircuit including a predistorter and a predistortion adjuster;

FIG. 5 shows a block diagram of a further embodiment of a transmitcircuit including a predistortion adjuster for obtaining updatedpredistortion coefficients;

FIG. 6 shows a block diagram of a further embodiment of a transmitcircuit including an envelope tracker; and

FIG. 7 shows a block diagram of a further embodiment of a transmitcircuit including an envelope tracker with an envelope shaping unit.

DETAILED DESCRIPTION

In the following, operation conditions and requirements of some mobilecommunication devices, in which the present invention may be used, willbe described. Some embodiments according to the invention provide goodperformance under the condition discussed in the following.

Mobile terminals often have to cope with changing environmentalconditions. The operating temperature range is typically between −10° C.up to 55° C. (according to 3GPP), while the supply voltage is typicallybetween 3.0V and 4.3V. The latter is determined by the battery dischargecharacteristic and voltage drop during transmit operation. In addition,the radiated power of a mobile terminal strongly depends on the antennaconditions, e.g. free space, talk position (antenna covered by hand orclose to head). The different antenna conditions result in differentload impedances effective at the power amplifier output. Typically, thepower amplifier has to cope with a wide range of load impedances. In thepast, however, most of the power amplifiers were optimized for 50 Ohmconditions. Also, in the power amplifier specification, special care wasnot taken regarding mismatch (despite requirements for ruggedness andstability). Especially, power into a mismatched load was neglected,which increased the effort for antenna and RF development (e.g.optimized post power amplifier matching and antenna matching). Since theradiated power was not satisfying, the network operator started todefine specific requirements, so called TRP (Total Radiated Power)requirements, especially driven by the US where the coverage is worsethan e.g. Europe.

Therefore, a need exists for an approach for adjusting a bias of a poweramplifier allowing to achieve a good trade-off between an improvedradiated performance, a low current consumption and the computationcomplexity.

Embodiments of the invention achieve the just-mentioned good trade-offby transforming an antenna impedance to an impedance at an input of anantenna tuner, wherein the input of the antenna tuner is coupled to anoutput of a power amplifier, and by providing a bias control signal foradjusting a bias of the power amplifier based on a determination of ameasure of a load impedance provided to the power amplifier by theantenna tuner. In this way, it is possible to avoid a degradation of theradiated performance during an impedance matching performed by theantenna tuner, so that the key parameters of the performance can bemaintained with comparatively low effort and/or comparatively lowcurrent consumption.

FIG. 1 shows a block diagram of an embodiment of a transmit circuit 100including an antenna tuner 130 and a bias controller 120. As shown inFIG. 1, the transmit circuit 100 comprises a power amplifier 110, theantenna tuner 130 and the bias controller 120. Here, the power amplifier110 is configured to amplify an RF input signal 105 to obtain an RFoutput signal 115. The antenna tuner 130 is configured to transform anantenna impedance to an impedance at an input of the antenna tuner 130,wherein the input of the antenna tuner 130 is coupled to an output ofthe power amplifier 110. Moreover, the bias controller 120 is configuredto control a bias of the power amplifier 110. The RF input signal 105may comprise a plurality of frequencies in specific frequency bands,such as defined by the UMTS standard (or may be switchable between aplurality of frequencies). Referring to the embodiment of FIG. 1, thebias controller 120 is configured to provide a bias control signal 125to adjust the bias of the power amplifier 110 based on a determinationof a measure of a load impedance provided to the power amplifier 110 bythe antenna tuner 130. Such a measure of the load impedance is, forexample, a quantity that is dependent on the load impedance. The RFoutput signal 115 obtained at the output of the power amplifier 110represents an amplified version of the RF input signal 105.

FIG. 2 shows a block diagram of a further embodiment of a transmitcircuit 200 including an antenna tuner 130 and a bias controller 220with an impedance determinator 222, a bias information provider 224 anda DCDC converter 226. Here, the transmit circuit 200 of FIG. 2essentially comprises the same blocks as the transmit circuit 100 ofFIG. 1. Therefore, identical blocks having similar implementationsand/or functions are denoted by the same numerals. Moreover, the biascontroller 220 and a power amplifier supply voltage 225, Vcc, of thetransmit circuit 200 shown in FIG. 2 may correspond to the biascontroller 120 and the bias control signal 125 of the transmit circuit100 shown in FIG. 1. Referring to the embodiment of FIG. 2, the transmitcircuit 200 comprises a power amplifier 110 for amplifying an RF inputsignal 105 based on the power amplifier supply voltage 225, Vcc, toobtain an RF output signal 115. The embodiment of FIG. 2 also includesan antenna tuner 130 for transforming an antenna impedance to animpedance at an input of the antenna tuner 130, wherein the input of theantenna tuner 130 is coupled to an output of the power amplifier 110. Ascan be seen in FIG. 2, the bias controller 220 of the transmit circuit200 comprises an impedance determinator 222, a bias information provider224 and a DCDC converter 226. Here, the impedance determinator 222 isconfigured to determine a measure 221,

_(L), of a load impedance provided to the power amplifier 110 by theantenna tuner 130, for example in the form of a reflection factor

_(L). The bias information provider 224 is configured to provide biasinformation 223 (e.g. a ramp voltage Vramp) based on the measure 221,

_(L), of the load impedance. Here,

_(L) is just a different representation of the load impedance asS-parameter considering a predetermined reference impedance. The DCDCconverter 226 is configured to adjust the supply voltage 225 of thepower amplifier 110 based on the bias information 223. In the embodimentof FIG. 2, the bias information provider 224 is configured to providethe bias information 223 such that a parameter of the RF output signal115 lies within a predefined range for a plurality of load impedancesdetermined by the impedance determinator 222. Here, the parameter of theRF output signal may correspond to an ACLR (Adjacent Channel LeakagePower Ratio) value, an EVM (Error Vector Magnitude) value or a saturatedpower value.

FIG. 3 shows a block diagram of a further embodiment of a transmitcircuit 300 including an antenna tuner 130 and a bias controller 320with an impedance information look-up table 324 (LUT B). As shown inFIG. 3, the transmit circuit 300 comprises a power amplifier system 310,the antenna tuner 130, the bias controller 320 and a directional coupler306. Here, the power amplifier system 310 and the bias controller 320 ofthe transmit circuit 300 shown in FIG. 3 may correspond to the poweramplifier 110 and the bias controller 120 of the transmit circuit 100shown in FIG. 1. In the embodiment of FIG. 3, a baseband generator 302(baseband generator ‘BB’) and an RF signal generator 304 (‘RF signalgeneration’) are also shown. The baseband generator 302 is configured togenerate a baseband signal 303, while the RF signal generator 304 isconfigured to generate an RF signal 305 on the basis of the basebandsignal 303. It can be seen in FIG. 3 that the power amplifier system 310is configured to receive the RF signal 305 from the RF signal generator304 to obtain an RF output signal 315. Here, the RF signal 305 receivedby the power amplifier system 310 and the RF output signal 315 output bythe power amplifier 310 as shown in the embodiment of FIG. 3 maycorrespond to the RF input signal 105 received by the power amplifier110 and the RF output signal 115 output by the power amplifier 110 asshown in the embodiment of FIG. 1. Moreover, the power amplifier system310 of FIG. 3 comprises a power amplifier 312 connected to an RFfrontend 314. Referring to FIG. 3, the transmit circuit 300 may bedivided into different parts, which may correspond to the poweramplifier system 310, an RF transceiver 340 and a tuner system 350. TheRF transceiver 340 comprises the RF signal generator 304, while thetuner system 350 comprises the antenna tuner 130 controlled by a tunercontroller 352. The tuner controller 352 of the tuner system 350 may beconfigured to receive a tuner control signal 351 provided by the RFtransceiver 340 for the tuner system 350.

According to embodiments, the directional coupler 306 of the transmitcircuit 300 is coupled to the output of the power amplifier 312 or tothe output of the RF frontend 314, such that it can be used to perform ameasurement of the load impedance provided to the power amplifier system310 (or to the power amplifier 312) by the antenna tuner 130.

Referring to the embodiment of FIG. 3, the antenna tuner 130 isconfigured to transform an antenna impedance (at an antenna 308) to animpedance at an input of the antenna tuner 130. Here, the input of theantenna tuner 130 is coupled to an output of the power amplifier system310. The bias controller 320 of the transmit circuit 300 comprises animpedance determinator 322, an impedance information look-up table 324(LUT B), a look-up table 326 (LUT A), a first digital-to-analogconverter (DAC) 328-1, a second digital-to-analog converter (DAC) 328-2and a DCDC converter 330. Here, the impedance determinator 322 isdenoted by ‘determine tuner input impedance

_(L)’, while the impedance information look-up table 324 and the look-uptable 326 are denoted by ‘LUT B: Store

_(L) data depending on frequency’ and ‘LUT A: Vcq=f(

_(L)); Vcc=f(

_(L))’, respectively. Moreover, the impedance determinator 322 and theDCDC converter 330 of FIG. 3 may correspond to the impedancedeterminator 222 and the DCDC converter 226 of FIG. 2.

According to the embodiment of FIG. 3, the directional coupler 306 isconfigured to provide a measurement signal 307 representing, forexample, a complex load impedance and forward the provided measurementsignal 307 to the impedance determinator 322 of the bias controller 320.The impedance determinator 322 may, in turn, be configured to determinethe measure 321,

_(L), of the (complex-valued) load impedance, such as a complex-valuedreflection factor

_(L), provided to the power amplifier 312 by the antenna tuner 130.Here, the measure 321,

_(L), of the load impedance obtained by the impedance determinator 322as shown in FIG. 3 may correspond to the measure 221,

_(L), of the load impedance obtained by the impedance determinator 222as shown in FIG. 2.

According to embodiments, the impedance determinator 322 may be part ofthe tuner system 350. Referring to FIG. 3, the impedance determinator322 is configured to provide the measure 321 of the load impedance forthe RF transceiver 340 or the impedance information look-up table 324.This is indicated in FIG. 3 by the arrow denoted by ‘report inputimpedance

_(L)’. The tuner controller 352 of the tuner system 350 as shown in FIG.3 may be configured to control the antenna tuner 130 for performing theantenna tuning (or impedance matching) based on the measure 321,

_(L), of the load impedance provided by the impedance determinator 322and on the tuner control signal 351 received from the RF transceiver340.

In the embodiment of FIG. 3, the impedance information look-up table 324of the bias controller 320 may be configured to store a plurality ofvalues of a measure of a load impedance for corresponding frequencies ofthe RF input signal (RF signal 305). Here, the storing of the impedancevalues (

_(L) data) is based on the impedance determination performed by theimpedance determinator 322. In addition, the bias controller 320 isconfigured to extract an individual value 325 of the measure of the loadimpedance from the impedance information look-up table 324 (LUT B).Here, the individual value 325 of the measure of the load impedance maycorrespond to a frequency of the RF input signal 305 such as in afrequency hopping mode. Moreover, the bias controller 320 is configuredto provide the bias control signal to adjust the bias of the poweramplifier 312 based on the individual value 325 of the measure of theload impedance extracted from the impedance information look-up table324. Accordingly, a currently measured impedance value 321 may be usedas the value 325 of the impedance measure if the look-up table 324 doesnot comprise a stored, previously measured impedance value, and astored, previously measured impedance value from the look-up table 324may be used as the value 325 of the impedance measure otherwise.

Referring to FIG. 3, the bias controller 320 may comprise a look-uptable 326 (LUT A) configured to store a plurality of bias voltage values(Vramp, Vcq) associated with corresponding values of the measure 321 ofthe load impedance for a plurality of load impedances provided to thepower amplifier 312 by the antenna tuner 130. In addition, the biascontroller 320 may be configured to extract an individual bias voltagevalue (Vramp; Vcq) from the look-up table 326 (LUT A), wherein theindividual bias voltage value (Vramp; Vcq) corresponds to a value 325 ofthe measure of the load impedance determined by the bias controller 320.The bias controller 320 is furthermore configured to provide the biascontrol signal to adjust the bias of the power amplifier 312 based onthe individual bias voltage value extracted from the look-up table 326.

For example, as depicted in the embodiment of FIG. 3, the biascontroller 320 is configured to extract a first and a second digitalbias voltage value 327-1, 327-2 from the look-up table 326 (LUT A) basedon the individual value 325 of the measure of the load impedanceextracted from the impedance information look-up table 324 (LUT B) orbased on a determined impedance information 321. Here, the first and thesecond digital bias voltage values 327-1, 327-2 may represent voltagesVcc or Vcq derived from a functional dependence on the measure of theload impedance or the reflection factor

_(L), respectively.

The first and the second digital bias voltage values 327-1, 327-2extracted from the look-up table 326 are converted into a first analogbias voltage value 329-1, Vramp, and a second analog bias voltage value329-2, Vcq, by a first and a second digital-to-analog converter 328-1,328-2, respectively. The DCDC converter 330 of the bias controller 320is configured to adjust the supply voltage 335, Vcc, of the poweramplifier 312 based on the first analog bias voltage value 329-1, Vramp.Here, the first analog bias voltage value 329-1 and the supply voltage335 in the embodiment of FIG. 3 essentially correspond to the biasinformation 223 and the supply voltage 225 in the embodiment of FIG. 2,respectively.

In other words, the DCDC converter 330 of the bias controller 320 isconfigured to adjust the supply voltage 335 of the power amplifier 312based on a bias information 329-1 determined by a selected entry of alook-up table 326 (LUT A), wherein the bias controller 320 is configuredto select an entry of the look-up table 326 based on the determinationof the measure 321 of the load impedance.

As shown in the embodiment of FIG. 3, the bias controller 320 isconfigured to provide a first bias control signal to adjust the supplyvoltage 335, Vcc, of the power amplifier 312. In addition, the biascontroller 320 is configured to further provide a second bias controlsignal to adjust an input-sided bias voltage 329-2, Vcq, to adjust aquiescent current of the power amplifier 312. Specifically, in theembodiment of FIG. 3, the power amplifier 312 is configured to amplifythe RF input signal 305 based on the supply voltage 335 and a quiescentcurrent adjusted by the input-sided bias voltage 329-2 (whichinput-sided bias voltage may, for example, adjust a gate bias or basebias of an amplifier transistor).

In the embodiment of FIG. 3, the bias controller 320 of the transmitcircuit 300 may be configured to control the bias of the power amplifier312 such that the ACLR value, the EVM value or the saturated power valueof the RF output signal lies within a predefined range for a pluralityof load impedances provided to the power amplifier 312 by the antennatuner 130. This ensures that even during the impedance matching processperformed by the antenna tuner, the key parameters for describing theradiated performance of the transmit circuit can be maintained.

Furthermore, the bias controller 320 may be configured to provide thebias control signal to adjust the supply voltage 335 of the poweramplifier 312 such that a maximum power capability of the poweramplifier 312 is altered (e.g., reduced) with improved impedancematching between the power amplifier 312 and the antenna tuner 130.

Thus, in the embodiment of FIG. 3, the antenna tuning system (tunersystem 350 including the impedance determinator 322) may provideinformation about the instantaneous input impedance (which is equal tothe power amplifier load impedance) of the antenna tuner (block 130).This impedance can, for instance, be derived by means of the directionalcoupler 306, which may be part of the antenna tuning system, or byevaluation of voltage levels which are measured at different impedancenodes inside the antenna tuning system. It is pointed out here that theantenna tuning system may provide capabilities to feed back thisinformation to the RF transceiver 340 or baseband IC (or to the biascontroller 320 including the impedance information look-up table 324).

The embodiment described with regard to FIG. 3 includes a DCDC converter(block 330). The DCDC converter can be used to set the supply voltageVcc of the power amplifier. According to embodiments, the DCDC convertercan be realized as a buck-only, a boost-only or a buck-boost converter.The supply voltage Vcc applied to the power amplifier 312 determines theoutput power capability of the power amplifier. The higher the supplyvoltage is, the higher the maximum output power of the power amplifieris. However, a high supply voltage also leads to a high battery currentdue to a lower conversion ratio of the DCDC converter. Thus, in oneembodiment the supply voltage Vcc is set to a level that should be aslow as possible to save battery current, but still high enough toensure, for example, good ACLR performance under all conditions, or tomeet other parameters like saturated power in GMSK (GaussianMinimum-Shift Keying) mode.

In one embodiment, the DCDC converter 330 is used to maintain anadequate power amplifier performance (e.g. to meet the ACLR or EVMtarget in 3G or LTE, or to meet the saturated power in GMSK mode) duringtime intervals in which the power amplifier, for example, may have tocope with a severe load VSWR (Voltage Standing Wave Ratio), that mayoccur when the antenna impedance is unknown or the tuning system has notyet set the desired power amplifier load impedance. Here, it is to benoted that the tuning process may take some time since the optimizationis an iterative process. During the time intervals in which the poweramplifier is potentially exposed to a higher load VSWR, the poweramplifier supply voltage (i.e. the DCDC converter output voltage) can beset to a higher level that, for instance, ensures good power amplifierlinearity even under severe antenna mismatch. When the power amplifierload impedance is being optimized by means of the antenna tuner 130, theDCDC converter 330 can simultaneously reduce the power amplifier supplyvoltage depending on the impedance information that is reported by theantenna tuning system. The power amplifier supply voltage can, forexample, be reduced if the impedance information reported by the antennatuner 130 indicates that the instantaneous antenna impedance isapproaching the desired target impedance. Such a reduction of the poweramplifier supply voltage can be made possible since according toembodiments of the invention, the power amplifier 312 needs less powerheadroom to cope with a mismatch caused by the antenna 308.

In addition to changing the power amplifier supply voltage that can beset by the DCDC converter, the power amplifier quiescent current canalso be used to scale the output power capability, as is shown in FIG.3. The quiescent current can either be set by a control voltage Vcq(shown in FIG. 3) or by an external reference current Iref (not shown)that can be provided by the RF transceiver.

According to the embodiment of FIG. 3, the bias controller 320 isconfigured to perform the following example steps. Initially, a firstbias control signal is provided to set the bias of the power amplifier312 to a comparatively high level during an initial transmission timeinterval or after an occurrence of a frequency change in a hoppingsequence. Then, the measure 321 of the load impedance provided to thepower amplifier 312 by the antenna tuner 130 is determined. Then, asecond bias control signal is provided that is different from the firstbias control signal to adjust the bias of the power amplifier to acomparatively lower level based on the determination of the measure 321of the load impedance for a consecutive time interval. Finally, the biaslevel of the power amplifier 312 is increased by providing an increasedbias control signal in response to a detection of a change of themeasure 321 of the load impedance that exceeds a predefined threshold.

In the embodiment described with regard to FIG. 3, the bias controller320 may be configured to set the bias of the power amplifier 312 to thecomparatively high level as long as a current measure of the loadimpedance is unavailable or as long as the load impedance presented tothe power amplifier 312 by the antenna tuner 130 has not reached apredetermined impedance region. Additionally, the bias controller 320may be configured to reduce the bias of the power amplifier 312 as soonas the load impedance presented to the power amplifier 312 by theantenna tuner 130 is brought into the predetermined impedance region bythe antenna tuner 130.

For example, the bias controller 320 may be configured to graduallyreduce the bias of the power amplifier 312 with a stepwise approximationof the load impedance presented to the power amplifier 312 by theantenna tuner 130 to a target impedance.

In the embodiment of FIG. 3, the bias controller 320 of the transmitcircuit 300 (or the impedance information look-up table 324) may beconfigured to receive frequency information 301. This frequencyinformation may indicate different frequencies of a hopping sequence ina frequency hopping mode of the transmit circuit. For example, theimpedance information look-up table 324 may be implemented to use thefrequency information 301 for storing the impedance measure data basedon a current frequency of the RF input signal in the hopping sequence.

Accordingly, the bias controller 320 may be configured to perform thesteps described with regard to the embodiment of FIG. 3 for eachfrequency of a hopping sequence. Here, each of the frequencies of thehopping sequence such as in the frequency hopping mode may be indicatedby the frequency information 301 received by the bias controller 320.Thus, in the embodiment of FIG. 3, the bias controller 320 is configuredto provide the bias control signal for the power amplifier 312 based onthe frequency information 301.

Furthermore, the bias controller 320 of the transmit circuit 300 may beconfigured to store, for a plurality of frequencies, measures of theload impedance determined by the bias controller 320, and to reuse thestored measures of the load impedance when returning to a frequencypreviously used in the hopping sequence.

In accordance with further embodiments, the procedure described abovewith respect to FIG. 3 may comprise the following example steps. Duringtime intervals in which the antenna tuner 130 does not provide thedesired power amplifier load impedance or target impedance (e.g. duringa first transmission slot, after a frequency change), the bias of thepower amplifier 312 that corresponds to the power amplifier supplyvoltage and/or power amplifier quiescent current is set to a(comparatively high) level that guarantees an adequate power amplifierperformance. The level can be set such that, for example, an ACLR or EVMtarget in 3G or LTE (Long Term Evolution) or a saturated power in GMSKmode will be met. It has been found that this can also be achieved undersevere mismatch conditions. When the power amplifier load impedance (orthe impedance at the input of the antenna tuner) starts to approach thedesired load impedance by means of the antenna tuner (due to theimpedance matching process), the power amplifier bias voltage is changedto account for the new power amplifier load conditions. The poweramplifier bias voltage is set based on the instantaneous impedance(measure 321 of the load impedance) that is reported by the tuner system(or the impedance determinator 322) to the RF transceiver (block 340) orbaseband IC (within block 302). Based on the reported impedanceinformation, the RF transceiver or baseband IC (or the bias controller320) sets a power amplifier bias in order to keep an adequate poweramplifier performance depending on the instantaneous antenna impedance.The antenna tuner system can, for example, report the impedanceinformation either by means of a serial interface such as MIPI RFFE (RFFrontend control interface) or by one or more analog voltages which, forinstance, depend on the magnitude and/or phase of the antenna tunerinput impedance. If the antenna tuner 130 reports a significantimpedance change, the bias of the power amplifier 312 is set to a securestate to guarantee an adequate power amplifier performance. When theantenna tuner feedback indicates better power amplifier load conditions,the bias of the power amplifier 312 is changed accordingly.

Optionally, if mobile devices operate in frequency hopping mode, thejust-mentioned procedure steps can be independently applied to eachhopping frequency. This can be achieved by a dedicated table (e.g. LUT Bin the embodiment of FIG. 3) that stores the impedance data for eachchannel.

In a further embodiment, the power amplifier load condition canalternatively be detected by a power amplifier sub-system that maycomprise the power amplifier 312 and the DCDC converter 330.

FIG. 4 shows a block diagram of a further embodiment of a transmitcircuit 400 including a predistorter 410 and a predistortion adjuster440. Here, the transmit circuit 400 of FIG. 4 essentially comprises thesame blocks as the transmit circuit 100 of FIG. 1. Therefore, identicalblocks having similar implementations and/or functions are denoted bythe same numerals. In the embodiment of FIG. 4, the predistorter 410 isconfigured to apply a predistortion to an input baseband signal 405 toobtain a predistorted baseband signal 415. Additionally, the transmitcircuit 400 of FIG. 4 may comprise an RF signal generator 420 and animpedance determinator 430. The impedance determinator 430 of FIG. 4 maycorrespond to the impedance determinator 222 of FIG. 2. The RF signalgenerator 420 is configured to provide the RF input signal 105 for thepower amplifier 110. Here, the RF signal generator 420 may be operativeon the predistorted baseband signal 415. The impedance determinator 430is configured to determine the measure 435 of the load impedanceprovided to the power amplifier 110 by the antenna tuner 130. Themeasure 435 of the load impedance provided by the impedance determinator430 as shown in FIG. 4 may correspond to the measure 221 of the loadimpedance provided by the impedance determinator 222 as shown in FIG. 2.Referring to the embodiment of FIG. 4, the predistortion adjuster 440 isconfigured to influence a predistortion applied by the predistorter 410based on the measure 435 of the load impedance. The predistortionapplied by the predistorter 410 may be influenced by using apredistortion control signal 445 output by the predistortion adjuster440. Optionally, the predistortion adjuster 440 of the transmit circuit400 may be configured to provide the predistortion control signal 445based on an error vector 401.

FIG. 5 shows a block diagram of a further embodiment of a transmitcircuit 500 including a predistortion adjuster 540 for obtaining updatedpredistortion coefficients. Here, the transmit circuit 500 of FIG. 5essentially comprises the same blocks as the transmit circuit 300 ofFIG. 3 and the transmit circuit 400 of FIG. 4. Therefore, identicalblocks having similar implementations and/or functions are denoted bythe same numerals. Moreover, the transmit circuit 500 shown in FIG. 5may comprise the bias controller 320 including the impedance informationlook-up table 324 (LUT B), the look-up table 326 (LUT A), the first andthe second digital-to-analog converters 328-1, 328-2 (DAC) and the DCDCconverter 330. The transmit circuit 500 of FIG. 5 furthermore comprisesa predistorter 510, an RF signal generator 520, an impedancedeterminator 530 and a predistortion adjuster 540, that may correspondto the predistorter 410, the RF signal generator 420, the impedancedeterminator 430 and the predistortion adjuster 440 of the transmitcircuit 400 of FIG. 4. Referring to the embodiment of FIG. 5, theimpedance determinator 530 is configured to determine the measure 535 ofthe load impedance from the measurement signal 307 representing, forexample, a complex load impedance output by the directional coupler 306.The measure 535 of the load impedance obtained by the impedancedeterminator 530 of FIG. 5 may correspond to the measure 435 of the loadimpedance obtained by the impedance determinator 430 of FIG. 4. Thesupply voltage Vcc, as shown in FIG. 5, may, for example, be provided bythe cooperation of the impedance information look-up table 324 (LUT B),the look-up table 326 (LUT A), the DAC 328-1 and the DCDC converter 330.

In the embodiment of FIG. 5, the predistortion adjuster 540 of thetransmit circuit 500 is configured to perform the following examplesteps. First, initial predistortion coefficients are provided for thepredistortion based on the measure 535 of the load impedance determinedby the impedance determinator 530 during an initial transmission timeinterval of a sequence of transmission time intervals. Subsequently,predistortion coefficients that are different from the initialpredistortion coefficients are set to influence the predistortionapplied by the predistorter 510 based on the measure 535 of the loadimpedance determined by the impedance determinator 530 for a consecutivetransmission time interval of the sequence of transmission timeintervals.

The predistortion adjuster 540 may be configured to provide the initialpredistortion coefficients independent from an error vector.Alternatively, the predistortion adjuster 540 may be configured toobtain updated predistortion coefficients based on the measure 535 ofthe load impedance and an error vector 501 describing a differencebetween a reconstructed baseband signal, obtained on the basis of an RFoutput signal 315 provided by the power amplifier system 310 (or by thepower amplifier 312), and the input baseband signal 505. Here, the errorvector 501 and the input baseband signal 505 of FIG. 5 may correspond tothe error vector 401 and the input baseband signal 405 of FIG. 4.Referring to FIG. 5, the error vector 501 to be (optionally) used by thepredistortion adjuster 540 can therefore be derived by comparing thereconstructed baseband signal (e.g. a down-converted, filtered RF outputsignal) with the input baseband signal (e.g. an I/O signal).

In the embodiment of FIG. 5, digital predistortion, which is awell-known approach, can be employed to improve the quality of thetransmit signal (e.g. the input baseband signal 505) in presence ofnon-linear effects caused by the power amplifier. It has been found thatin accordance with the embodiment of FIG. 5, the knowledge of theantenna impedance can be used to enhance the predistortion algorithm. Anunderlying concept of the predistortion applied by the predistorter 510is that the power amplifier output signal or the reconstructed basebandsignal is compared (e.g. after a down-conversion, filtering etc.) withthe input or modulated baseband signal (e.g. an I/O signal) that is usedto generate the RF signal. The result of the comparison is an errorvector 501 that is a function of the amplitude and phase error caused bythe nonlinearities in the power amplifier 312. Based on this errorvector, predistortion or correction coefficients can be generated by thepredistortion adjuster 540 that can, in turn, be used by thepredistorter 510 to predistort the baseband signal. It is to be notedthat the correction coefficients depend on the antenna impedance sincethe AMAM and AMPM characteristics of the power amplifier 312 depend onthe load impedance. Typically, the predistortion algorithm performed bythe predistorter 510 is able to compensate a variable antenna impedance;however this normally requires some iterations. In case of the antennatuning by the antenna tuner 130, the power amplifier load impedancecontinuously changes since the antenna tuning system tries to optimizethe power amplifier load impedance. Therefore, in order to align theantenna tuning algorithm with the predistortion algorithm, the proceduredescribed above with respect to FIG. 5 can be employed.

Accordingly, the following example steps can be performed by thecooperation of the impedance determinator 530, the predistortionadjuster 540 and the predistorter 510. First, the initial predistortioncoefficients are provided that may depend on a first power amplifierload impedance or another parameter that depends on the power amplifierload impedance. Then, during the antenna tuning, the predistortioncoefficients are set by means of the calculated error vector and byknowledge of the instantaneous power amplifier load impedance. In animplementation according to FIG. 5, the predistortion can be disabled incase of a fast changing power amplifier load impedance, or genericpredistortion coefficients can be used that are less sensitive tochanges of the power amplifier load impedance. In a furtherimplementation, the predistortion coefficients can be corrected tominimize the error caused by a changed power amplifier load impedance.

FIG. 6 shows a block diagram of a further embodiment of a transmitcircuit 600 including an envelope tracker 640. As shown in FIG. 6, thetransmit circuit 600 comprises a power amplifier 610, a bias provider620, an impedance determinator 630 and an envelope tracker 640. Theenvelope tracker 640 is configured to determine an envelope of atransmit signal 605 and provide bias information 645 based on theenvelope of the transmit signal 605. The power amplifier 610 isconfigured to obtain an RF output signal 615 based on the transmitsignal 605. Similar to the embodiments described above with regard toFIGS. 1 to 5, the power amplifier 610 may be configured to be operativeon an RF input signal 607 that is derived from the transmit signal 605,wherein the RF input signal 607 may comprise a plurality of frequenciesin specific frequency bands based on the UMTS standard (or may beswitchable between a plurality of frequencies). The bias provider 620 isconfigured to provide a bias 625 for the power amplifier 610 based onthe bias information 645. Here, the bias information 645 may represent aramp voltage for indicating the bias 625, wherein the bias 625 mayrepresent a supply voltage for the power amplifier 610. The biasprovider 620 may, for example, be implemented as a DCDC converterconfigured to provide the bias 625 or the power amplifier supply voltagebased on the bias information 645 or the ramp voltage at the output ofthe envelope tracker 640. The impedance determinator 630 is configuredto determine a measure 635 of a load impedance of a load 650 coupled toan output of the power amplifier 610. As opposed to the embodimentsdescribed above with regard to FIGS. 1 to 5, the output of the poweramplifier 610 in the embodiment of FIG. 6 is not coupled to an input ofan antenna tuner but to the load 650. The measure 635 of the loadimpedance is, for example, a quantity that is dependent on the loadimpedance, such as a complex-valued reflection factor

_(L). Referring to the embodiment of FIG. 6, the envelope tracker 640 isconfigured to adapt the provision of the bias information 645 based onthe measure 635 of the load impedance. Thereby, the envelope trackingperformed by the envelope tracker 640 can be significantly improvedbased on the knowledge of the impedance measure determined by theimpedance determinator 630.

FIG. 7 shows a block diagram of a further embodiment of a transmitcircuit 700 including an envelope tracker 740 with an envelope shapingunit 744. The transmit circuit 700 of FIG. 7 comprises a power amplifiersystem 710, an impedance determinator 730, an envelope tracker 740 andan antenna tuner 750. Here, the blocks 710, 730 and 740 as shown in FIG.7 may correspond to the blocks 610, 630 and 640 of FIG. 6. In theembodiment of FIG. 7, the transmit circuit 700 comprises a basebandgenerator 702, an RF signal generator 706, the power amplifier system710 and the antenna tuner 750, thereby defining a main signal path. Thebaseband generator 702 is configured to generate a baseband signal 705as the transmit signal. The RF signal generator 706 is configured togenerate an RF signal 707 from the baseband signal 705. The poweramplifier system 710 may comprise a power amplifier 712 and an RFfrontend 714. As shown in FIG. 7, the power amplifier system 710 (or thepower amplifier 712) is configured to obtain an RF output signal 715based on the transmit signal 705. In addition, the input of the antennatuner 750 is coupled to an output of the power amplifier 710. Theantenna tuner 750 is configured to transform an antenna impedance (at anantenna 308) to an impedance at the input of the antenna tuner 750.Thus, by the cooperation of the baseband generator 702, the RF signalgenerator 706, the power amplifier system 710 and the antenna tuner 750,a main signal path is provided.

Additionally, the impedance determinator 730 is configured to determinethe measure 735 of the load impedance based on a measurement signal 307(e.g. a signal representing a complex load impedance) obtained by adirectional coupler 306. Similar to the embodiments of FIGS. 1 to 5, theinput-sided bias voltage (Vcq) of the power amplifier 712 can becontrolled by the cooperation of an impedance information look-up table324, a look-up table 326 and a digital-to-analog converter 328-2 basedon the determined impedance measure 735. Here, the components 306, 324,326 and 328-2 of FIG. 7 may correspond to the components of FIG. 3,which are referenced by the same numerals. In addition, the transmitcircuit 700 of FIG. 7 may comprise an envelope tracker 740 for providingbias information 745, such as a ramp voltage Vramp, based on theenvelope of the transmit signal 705. The transmit circuit 700 mayfurthermore comprise a DCDC converter 720 for providing a bias 725, suchas the supply voltage Vcc of the power amplifier 712 based on the biasinformation 745, Vramp. In the embodiment of FIG. 7, the DCDC converter720 may be implemented as a special DCDC converter, such as an ET(Envelope Tracking) DCDC converter. The envelope tracker 740 and theDCDC converter 720 shown in FIG. 7 may correspond to the envelopetracker 640 and the bias provider 620 shown in FIG. 6. Together, theenvelope tracker 740 and the DCDC converter 720 of FIG. 7 define anenvelope path within the transmit circuit 700.

For the envelope path, the envelope tracker 740 may comprise an envelopegeneration unit 742, an envelope shaping unit 744 and adigital-to-analog converter 748. The envelope generation unit 742 isconfigured to generate the envelope 743 of the transmit signal from thebaseband signal 705 provided by the baseband generator 702. The envelopeshaping unit 744 is configured to shape the envelope 743 of the transmitsignal by using a shaping characteristic that depends on the measure 735of the load impedance provided to the power amplifier 712 by the antennatuner 750. At the output of the envelope shaping unit 744, an analogsignal 747 representing the shaped envelope may be obtained, which canbe converted into a digital signal representing the bias information745, Vramp, by a digital-to-analog converter 748. The supply voltage Vccat the output of the DCDC converter 720 is provided based on the digitalsignal 745 obtained from the DAC 748. Thus, by the use of the envelopetracker 740 and the DCDC converter 720, a shaping characteristic forshaping the envelope of the transmit signal can be improved based on theknowledge of the determined measure 735 of the load impedance provided,such as during an impedance matching. In accordance with the embodimentsdescribed above with regard to FIGS. 6 and 7, an improved envelopetracking (ET) is provided within a transmit circuit. Specifically, ithas been found that the knowledge of the antenna impedance can be usedto enhance the envelope tracking algorithm performed by the envelopetracker. By such an envelope tracking approach, an improved RF amplifierdesign can be implemented in which the power supply voltage applied tothe power amplifier can be constantly (or variably) adjusted to ensurethat the amplifier is operating at peak efficiency for the giveninstantaneous output power requirements.

In the envelope tracking according to the embodiment of FIG. 7, thepower supply voltage to the power amplifier is not constant. Instead,the power supply voltage to the power amplifier can be changed dependingon the instantaneous envelope of the (modulated) baseband signal. Inembodiments, the envelope of the (modulated) baseband signal can becalculated by means of a CORDIC (Coordinate Rotation Digital Computer)algorithm. This can be followed by a delay adjustment to compensate adifferent delay in the main signal path (RF signal generation path) andthe envelope path. Then, the envelope signal can be shaped(predistorted) and finally digital-to-analog converted. The thusobtained signal can be fed to the ET DCDC converter (special DCDCconverter) that generates the variable power supply for the poweramplifier. Similar to predistortion as described with regard to FIGS. 4and 5, the envelope shaping characteristic can be applied depending onthe power amplifier load impedance. As opposed to known envelopetracking approaches, that are based on a feed-forward implementation,the power amplifier output signal can advantageously be used to adaptthe shaping function or shaping characteristic applied by the envelopeshaping unit. Since the power amplifier load impedance is known by theimpedance determinator, the shaping function does not need to compromiseall the potential power amplifier load impedances.

According to embodiments, this knowledge of the load impedance can beused to optimize the shaping characteristic and thus to reduce thebattery current consumption. In addition, the shaping characteristic canbe adapted whenever a new power amplifier load impedance is effectivedue to the antenna tuning algorithm.

Some embodiments according to the invention provide a better performancethan conventional 3G mobile devices that use an isolator to keep goodACLR performance under antenna mismatch. The isolator resolved thelinearity degradation under antenna mismatch, but had severe impact onsize and cost. The increasing number of bands aggravated the cost andsize disadvantage of the isolator approach. As a result, the isolatorwas removed from most of the designs and eliminated by other approachesthat shall also provide a load-insensitive behavior. Today, balancedpower amplifiers are the most important class of load insensitive poweramplifier solutions. There are some design variants depending on thepower amplifier supplier, but all implementations rely on a 90 deghybrid as a core element to reduce the load sensitivity. One majordisadvantage of each balanced power amplifier is that the loadinsensitivity is gained at the expense of a lower power amplifierefficiency due to additional losses caused by the hybrid network. Thepeak efficiency of a balanced amplifier is typically in the range of35-37%, whereas a single ended power amplifier is more than 40%.

Some embodiments according to the invention provide a better tradeoffbetween efficiency and complexity than amplifiers having more headroom.It has been found that it is often a less effective approach to use asingle-ended amplifier with more headroom of linear output power. Due tothe extra linear power, the ACLR degradation under mismatch is reduced.The advantages compared to a balanced power amplifier are less complexhardware, which allows more cost effective and smaller size solutions.However, the impact on efficiency is even more severe than in case of abalanced power amplifier if the same ALCR performance under mismatch forboth architectures is presumed.

Some embodiments according to the invention provide a better performancethan conventional antenna tuning systems. The use of an antenna tunerhas become more and more popular. The antenna tuner is basically animpedance matching network with at least two different states thattransforms the antenna impedance to an impedance value at the input ofthe tuner which is closer to the optimum load impedance of the poweramplifier (normally 50 Ohm). The antenna tuner reduces the mismatch lossat the antenna input in order to increase the power delivered to theantenna sub-system which shall consist of a tuner and an antenna. Itdoes not change the antenna efficiency itself which is the ratio of theactually radiated power and the power delivered to the antennasub-system. Thus, the main effect of an antenna tuner is the increase ofthe power delivered to the antenna sub-system. Nevertheless, dependingon the mismatch loss, the antenna tuner can achieve a 1 . . . 2 dBincrease of radiated power. Today's antenna tuners provide manydifferent impedance states and can cover a big tuning range which ishere defined as the antenna impedance range which can be mapped to a 50Ohm impedance at the input of the tuner.

However, usually, antenna tuners operate in an open loop way. This meansthat the tuner state is set depending on one or more instantaneous andknown states of the mobile terminal such as the transmit/receivefrequency and mechanical state of the phone (e.g. slider position). Theantenna impedance itself is usually unknown. Thus, the achievableimprovement is most of the time not optimum since the actual antennaimpedance changes due to the user specific handling of the mobileterminal (e.g. how the device is touched during the call). It has beenfound that it is possible to improve the benefit of the antenna tuner ifthe tuner state can be set depending on the actual antenna impedance.This requires additional means that detect the instantaneous antennaimpedance or tuner input impedance that is a function of the antennaimpedance. According to some embodiments, the power amplifier design canbe relaxed (which also means lower cost), since in a closed loop antennatuning system, the load VSWR effective at the power amplifier output isreduced and prevents severe VSWR conditions. However, this can only beachieved if the antenna tuning system has sufficient time to optimizethe power amplifier load impedance. Usually, this condition is, forinstance, not fulfilled at a first transmit interval where the antennaimpedance is unknown, after a frequency change or after a transmissionidle phase that is long enough so that antenna impedance may change. Aslong as the tuner system does not provide the desired power amplifierload conditions, the power amplifier may have to cope with severe loadVSWR. However, embodiments of the invention, offer a solution how tokeep the key parameters (e.g. ACLR or EVM for a linear system, or powerdelivered to the antenna for a GMSK based mobile terminal) even duringtime intervals for which the antenna tuner cannot provide the desiredpower amplifier load impedance.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, that are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitionary.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processor, for example a computer, or aprogrammable logic device, configured to or adapted to perform one ofthe methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

In summary, embodiments of the invention offer an attractive solutionfor a problem which occurs in mobile terminals based, for example, on8PSK/GMSK (8-Phase Shift Keying/Gaussian Minimum Shift Keying), WCDMA orLTE, which employ an antenna tuner to improve the radiated performance.

Embodiments of the invention provide a procedure for setting the poweramplifier bias depending on an impedance value which is reported by thetuner system.

Embodiments of the invention have the advantage that the impedanceinformation can be used to change the predistortion characteristic.

According to embodiments of the invention, it is possible to use theimpedance information to change the shaping characteristic in anenvelope tracking system.

What is claimed is:
 1. A transmit circuit, comprising: an envelopetracker configured to determine an envelope of a transmit signal andprovide bias information based on the determined envelope of thetransmit signal; a power amplifier configured to generate an RF outputsignal based on the transmit signal; a bias provider configured toprovide a bias for the power amplifier based on the bias information;and an impedance determinator configured to determine a measure of aload impedance of a load coupled to an output of the power amplifier;wherein the envelope tracker is configured to adapt the bias informationbased on the measure of the load impedance.
 2. The transmit circuit ofclaim 1, further comprising: a baseband generator configured to generatea baseband signal as the transmit signal; an RF signal generatorconfigured to generate an RF signal from the baseband signal; and anantenna tuner configured to transform an antenna impedance to the loadimpedance at an input of the antenna tuner, wherein the input of theantenna tuner is coupled to an output of the power amplifier; whereinthe envelope tracker further comprises: an envelope generation unitconfigured to generate the envelope of the transmit signal from thebaseband signal; and an envelope shaping unit configured to shape theenvelope of the transmit signal by using a shaping characteristic thatdepends on the measure of the load impedance provided to the poweramplifier by the antenna tuner.
 3. The transmit circuit of claim 1,wherein the bias comprises a first bias control signal to adjust asupply voltage of the power amplifier.
 4. The transmit circuit of claim3, wherein the bias comprises a second bias control signal to adjust aquiescent current of the power amplifier.
 5. The transmit circuit ofclaim 3, wherein the power amplifier is configured to amplify thetransmit signal based on the supply voltage and a quiescent current ofthe power amplifier.
 6. The transmit circuit of claim 1, wherein thebias provider comprises an impedance information look-up tableconfigured to store a plurality of values of a measure of a loadimpedance for corresponding frequencies of the transmit signal.
 7. Thetransmit circuit of claim 6, wherein the bias provider is configured toextract an individual value of the measure of the load impedance fromthe impedance information look-up table, wherein the individual value ofthe measure of the load impedance corresponds to a frequency of thetransmit signal in a frequency hopping mode, and wherein the biasprovider is configured to provide a bias control signal as the bias toadjust the bias of the power amplifier based on the individual value ofthe measure of the load impedance extracted from the impedanceinformation look-up table.
 8. The transmit circuit of claim 1, whereinthe bias provider further comprises a DCDC converter configured toadjust a supply voltage of the power amplifier based on bias informationdetermined by a selected entry of a look-up table, wherein the biasprovider is configured to select an entry of the look-up table based onthe determination of the measure of the load impedance.
 9. The transmitcircuit of claim 1, further comprising a directional coupler configuredto perform a measurement of the load impedance provided to the poweramplifier by an antenna tuner.
 10. The transmit circuit of claim 1,wherein the bias provider is configured to control the bias of the poweramplifier such that an adjacent channel leakage power ratio value, anerror vector magnitude value or a saturated power value of the RF outputsignal lies within a predefined range for a plurality of load impedancesprovided to the power amplifier by an antenna tuner.
 11. The transmitcircuit of claim 1, wherein the bias provider is configured to provide abias control signal as the bias to adjust a supply voltage of the poweramplifier such that a maximum power capability of the power amplifier isreduced with improved impedance matching between the power amplifier andan antenna tuner.
 12. The transmit circuit of claim 1, wherein the biasprovider is configured to provide a first bias control signal as thebias to set the bias of the power amplifier to a comparatively highlevel during an initial transmission time interval or after anoccurrence of a frequency change in a hopping sequence, determine themeasure of the load impedance provided to the power amplifier by theantenna tuner, provide a second bias control signal as the bias that isdifferent from the first bias control signal to adjust the bias of thepower amplifier to a comparatively lower level based on thedetermination of the measure of the load impedance for a consecutivetime interval, and increase the bias level of the power amplifier byproviding an increased bias control signal as the bias in response to adetection of a change of the measure of the load impedance that exceedsa predefined threshold.
 13. The transmit circuit of claim 12, whereinthe bias provider is configured to set the bias of the power amplifierto the comparatively high level as long as a current measure of the loadimpedance is unavailable or as long as the load impedance presented tothe power amplifier by the antenna tuner has not reached a predeterminedimpedance region, and wherein the bias provider is configured to reducethe bias of the power amplifier as soon as the load impedance presentedto the power amplifier by an antenna tuner is brought into thepredetermined impedance region by the antenna tuner.
 14. The transmitcircuit of claim 13, wherein the bias provider is configured togradually reduce the bias of the power amplifier with a stepwiseapproximation of the load impedance presented to the power amplifier bythe antenna tuner to a target impedance.
 15. The transmit circuit ofclaim 12, wherein the bias provider is configured to provide the firstbias control signal as the bias to set the bias of the power amplifierto the comparatively high level during the initial transmission timeinterval or after the occurrence of the frequency change in a hoppingsequence, determine the measure of the load impedance provided to thepower amplifier by an antenna tuner, provide the second bias controlsignal as the bias that is different from the first bias control signalto adjust the bias of the power amplifier to the comparatively lowerlevel based on the determination of the measure of the load impedancefor the consecutive time interval, and increase the bias level of thepower amplifier by providing an increased bias control signal as thebias in response to the detection of the change of the measure of theload impedance that exceeds a predefined threshold, for each frequencyof a hopping sequence.
 16. The transmit circuit of claim 15, wherein thebias provider is configured to store, for a plurality of frequencies,measures of the load impedance determined by the bias controller, and toreuse the stored measures of the load impedance when returning to afrequency previously used in the hopping sequence.
 17. The transmitcircuit of claim 1, further comprising: a predistorter configured toapply a predistortion to an input baseband signal to obtain apredistorted baseband signal; an RF signal generator configured toprovide the transmit signal for the power amplifier based on thepredistorted baseband signal; and a predistortion adjuster configured toinfluence the predistortion applied by the predistorter based on themeasure of the load impedance.
 18. A method for adapting the provisionof bias information, the method comprising: determining an envelope of atransmit signal; providing bias information based on the determinedenvelope of the transmit signal; obtaining an RF output signal based onthe transmit signal using a power amplifier; providing a bias for thepower amplifier based on the bias information; determining a measure ofa load impedance of a load coupled to an output of the power amplifier;adapting the bias information based on the measure of the loadimpedance; and generating the envelope of the transmit signal from abaseband signal, wherein generating the envelope of the transmit signalfurther comprises shaping the envelope of the transmit signal using ashaping characteristic that depends on the measure of the loadimpedance.